First - lower the hook a notch. They're a little high and will become a problem with heavy weights. You'll also be able to get set under the bar better that way.
Second - you need to tighten up, especially your upper back. Read the thoracic extension article. The relative softness is contributing to you getting off balance and bouncing around on a couple of the reps.
Third - keep your knees locked out where they're supposed to be. You're a little close on depth.